Re: [Qemu-devel] [PATCH 00/21][RFC] postcopy live migration

[Orit Wasserman <owasserm@redhat.com>]

On 12/30/2011 12:39 AM, Anthony Liguori wrote:
> On 12/28/2011 07:25 PM, Isaku Yamahata wrote:
>> Intro
>> =====
>> This patch series implements postcopy live migration.[1]
>> As discussed at KVM forum 2011, dedicated character device is used for
>> distributed shared memory between migration source and destination.
>> Now we can discuss/benchmark/compare with precopy. I believe there are
>> much rooms for improvement.
>>
>> [1] http://wiki.qemu.org/Features/PostCopyLiveMigration
>>
>>
>> Usage
>> =====
>> You need load umem character device on the host before starting migration.
>> Postcopy can be used for tcg and kvm accelarator. The implementation depend
>> on only linux umem character device. But the driver dependent code is split
>> into a file.
>> I tested only host page size == guest page size case, but the implementation
>> allows host page size != guest page size case.
>>
>> The following options are added with this patch series.
>> - incoming part
>>    command line options
>>    -postcopy [-postcopy-flags<flags>]
>>    where flags is for changing behavior for benchmark/debugging
>>    Currently the following flags are available
>>    0: default
>>    1: enable touching page request
>>
>>    example:
>>    qemu -postcopy -incoming tcp:0:4444 -monitor stdio -machine accel=kvm
>>
>> - outging part
>>    options for migrate command
>>    migrate [-p [-n]] URI
>>    -p: indicate postcopy migration
>>    -n: disable background transferring pages: This is for benchmark/debugging
>>
>>    example:
>>    migrate -p -n tcp:<dest ip address>:4444
>>
>>
>> TODO
>> ====
>> - benchmark/evaluation. Especially how async page fault affects the result.
> 
> I'll review this series next week (Mike/Juan, please also review when you can).
> 
> But we really need to think hard about whether this is the right thing to take into the tree.  I worry a lot about the fact that we don't test pre-copy migration nearly enough and adding a second form just introduces more things to test.
>
> It's also not clear to me why post-copy is better.  If you were going to sit down and explain to someone building a management tool when they should use pre-copy and when they should use post-copy, what would you tell them?

Start with pre-copy , if it doesn't converge switch to post-copy  

Orit
> 
> Regards,
> 
> Anthony Liguori
>